test: findUserArg has no unit tests

`findUserArg` in `singleContainer.ts` is exported but has no test coverage.
No test file currently imports from `singleContainer`.

Proposing to add `src/test/singleContainer.test.ts` covering these cases:
- ['-u', 'myuser'] -> 'myuser'
- ['--user', 'myuser'] -> 'myuser'
- ['--user=myuser'] -> 'myuser'
- ['--user', 'a', '-u', 'b'] -> 'b' (last wins)
- [] -> undefined

Would this be helpful?

首先阅读 singleContainer.ts 中的 findUserArg，并查看附近测试所采用的约定。为列出的五种参数情况添加 src/test/singleContainer.test.ts，然后使用仓库现有的测试命令运行该测试文件。当每个预期的用户名或 undefined 结果都得到覆盖并通过时，即表示完成。

由索引模型根据 Issue 内容生成。